A variety of different devices have previously been proposed for in-vivo imaging of a subject's eye. Such devices are typically used by a clinician or physician to look for abnormalities (congenital or acquired) and symptoms of disease.
One such previously proposed device is the so-called “ophthalmoscope”. Commonly available opthalmoscopes range from relatively simple pocket-sized opthalmoscopes such as the Welch Allyn PocketScope™ Ophthalmoscope, to more complex devices such as the Welch Allyn Panoptic™ Ophthalmoscope (each of which are available from Welch Allyn Inc., Corporate Headquarters, 4341 State Street Road, Skaneateles Falls, N.Y. 13153-0220, USA and viewable at: www.welchallyn.com). In general terms, such devices allow an operator to shine a light into a subject's eye to illuminate the retina whilst the operator looks for abnormalities and symptoms of disease.
A more sophisticated device for imaging the retina of a subject is the so-called Scanning Laser Ophthalmoscope (often referred to as an “SLO”). The SLO is more accurate than a traditional ophthalmoscope and provides a greater field of view of the retina. The SLO comprises a laser source and a series of vertical and horizontal mirrors that can be operated to scan the laser over a subject's retina to generate a raster image that can be displayed on a conventional television monitor.
Whilst the SLO is able to image the retina in real time, reflections from the eye, astigmatism and higher order aberrations introduced by the cornea, tear film, lens and eye movements, can cause problems with the data produced by the device, resulting in poorer images. To address these issues, a device known as an Adaptive Optics Scanning Laser Ophthalmoscope (AOSLO) has more recently been proposed. This device uses adaptive optics to remove optical aberrations from images obtained with an SLO. In particular, in an AOSLO, a laser is collimated and then reflected off of a beam-splitting mirror. As with a conventional SLO, light is passed through both a horizontal and a vertical scanning mirror before and after the eye is scanned to align the moving beam for eventual retinal raster images of the retina. Additionally, the light is reflected off of a deformable mirror before and after exposure to the eye to compensate for optical aberrations. The laser enters the eye through the pupil to illuminate the region it has been focused onto and light reflected back passes to a beam splitter where it is directed simultaneously toward a photomultiplier tube (PMT) and toward a Shark-Hartmann wavefront sensor array. The light going toward the photomultiplier is focused through a confocal pinhole to remove light not reflecting off of the plane of interest and is then recorded in the PMT. Light directed to the wavefront sensor array is split up by the lenslets in the array and then recorded onto a Charge-coupled device (CCD) camera for detection of optical aberrations. These aberrations are optically compensated for by using the deformable mirror to increase lateral and axial resolution.
Another commonly used technique is called Optical Coherence Tomography (OCT). OCT provides a powerful clinical tool for monitoring retinal physiology in patients, and utilises low coherence interferometry to differentiate tissues within the eye and create a cross section of a living patients' retina non-invasively. OCT provides better axial resolution than AOSLO, however AOSLO represents tends to provide better translational resolution than OCT and can thus be used to track minor lateral physical changes, such as the effects of eye movements on the retina. A combination of AOSLO and OCT has also recently been proposed, which combination should provide, at high speed, three dimensional images of individual cone cells and an illustration of the overall cone mosaic near the fovea of a subject's eye.
Whilst these devices are all of use in imaging the eye of a subject, a principal problem with SLO, AOSLO and OCT devices is that they are relatively expensive, typically in the order of tens of thousands of pounds. The effect of this is that such devices tend not to be available to individual or small groups of practitioners, and instead tend to be limited to larger organisations, such as hospitals.
One technique that might be employed to address this issue is broadly similar to a technique that is commonly referred to as “structured illumination microscopy”. Structured illumination microscopy enables optical sectioning of a three-dimensional (3D) object resulting in images similar to those obtained using a confocal scanning microscope. It has also been used for enhanced lateral resolution, allowing superresolution beyond the diffraction limit. In the depth-resolving case, the basic principle between structured illumination microscopy and confocal microscopy is similar, namely that only planes that are in focus are imaged efficiently and out-of-focus planes contribute significantly less to the image.
However they are fundamentally different optical systems. Structured illumination microscopy has the advantage of being an optically simple technique that does not require laser illumination nor scanning of the beam or sample. The non-scanning configuration of the structured illumination microscope and the absence of a laser source enable a simple optical set up to be used that has minimal moving parts, and thus has potential for cost-effectiveness and robustness.
Another drawback of the confocal microscope is that the detector pinhole rejects light in order to achieve axial sectioning, and in practice, especially in the ophthalmic imaging case of the confocal Scanning Laser Ophthalmoscope (SLO), trade offs have to be made between pinhole size and confocality, thus limiting the axial sectioning capabilities of the device. Structured illumination microscopy does not reject any light and can therefore image the sample more efficiently.
In structured illumination microscopy, the sample is illuminated with a sinusoidal pattern along one of its lateral dimensions. For weak objects, it has been shown that it is only the zero-order spatial frequency that does not attenuate with defocus. As it is possible with sinusoidal illumination to recover an image in which the zero-order term is absent; all remaining spatial frequencies tend to attenuate with defocus thereby providing that the in-focus plane is the one that contributes most significantly to the image obtained. A drawback of this technique is that it is necessary to acquire three successive images with the sinusoidal pattern displaced by phases of ⅔π and −⅔π with respect to the first image. From these three images, an optically sectioned image of the sample can be obtained.
Whilst this approach appears useful, in the context of imaging non-stationary objects the requirement for multiple images proves problematic. This is particularly the case in the context of in-vivo retinal imaging where involuntary and voluntary tremors and saccades of the eye result in a typically continuously moving sample.
Another problem to be addressed is the manner in which the retina, in this particular example is to be illuminated. In particular in one technique known as “grid projection”, the frequency of the sinusoid needs to be carefully controlled so that the resultant frequency of the light illuminating the retina is in the region of 500 cycles per degree. However, a problem with this approach is that the optics of the eye does not transmit a sinusoid with a frequency above about 60 cycles per degree.
One way to address this last problem is to illuminate the retina with coherent sources that are allowed to interfere and thereby generate fringes on the retina. However, if coherent sources are used, all layers of the eye may an equal contribution and hence the ability to image the retina in three-dimensions is lost.
One previously proposed attempt to resolve these issues is disclosed in US2009/0046164. In this patent application the system disclosed is primarily intended to provide lateral superresolution (i.e. lateral resolution beyond the Rayleigh limit), and hence axial resolution (or in other words, three-dimensionality) is not of concern. This patent discloses the use of grid projection techniques using incoherent light, and fringe projection techniques using coherent laser light. However, incoherent grid projection techniques cannot function as a means to image the retina in-vivo due to the aforementioned transmission limit of around 60 cycles per degree (it would, of course, function adequately when used to image a sample in-vitro), and fringe projection techniques using coherent laser sources cannot provide axial resolution. In addition, in this patent application the aforementioned problem with a subject moving between successive images is countered by two different techniques, in one technique where the movement concerned is anticipated, phase shifts are estimated a priori. In another technique where movements cannot be anticipated, an algorithm is employed to estimate phase shifts a posteriori from peaks in the Fourier transform. As will be appreciated, in either case the accuracy of the resulting image is only as good as the phase shift estimations.
The present invention has been devised with the foregoing problems in mind.